1. Field of the Invention
The present invention relates to a polyester comprising ethylene terephthalate units as the main constituting component, which is suitable for production of e.g. stretch blow molded products required to have heat resistance. Further, the present invention relates to a process for producing such a polyester.
2. Discussion of Background
A polyester (hereinafter sometimes referred to as PET) comprising ethylene terephthalate units as the main constituting component, is widely used for containers for e.g. foods or beverages. In particular, as a container for a beverage required to be sterilized and filled under heating, such as mineral water, it is common to employ one obtained by producing a bottomed tubular preform by injection molding, then heating and softening it by e.g. an infrared ray heater, putting it in a mold, followed by stretch blow molding to obtain a container having a predetermined shape, and further applying heat treatment thereto to improve the heat resistance. Heretofore, for this application, PET produced by means of a germanium catalyst has been mainly employed. Especially in an application where high heat resistance is required, PET produced by means of an antimony catalyst has not been employed, as the heat resistance has been inadequate. The reason is that with PET produced by means of an antimony catalyst, the crystallization rate is high, and crystallization is likely to take place during the heating of the preform, whereby uniform stretch blow molding can hardly be carried out. As a method to avoid crystallization during the heating, it is conceivable to shorten the heating time of the preform, but by such a method, the preform may not sufficiently be heated, whereby the molecular motion is suppressed, and no adequate crystal orientation or relaxing of an orientation strain can take place during the subsequent heat treatment, whereby no adequate heat resistance will be obtained.
As another method, it is conceivable to reduce the crystallization rate of the resulting PET by employing a large amount of the copolymer component during the production of PET. However, in such a method, even if a molded product obtained by stretch blow molding, is subjected to heat treatment, the crystal orientation and relaxing of an orientation strain can not adequately proceed as they are hindered by the copolymer component, whereby improvement in the heat resistance tends to be also inadequate.
An antimony catalyst is inexpensive as compared with a germanium catalyst. Accordingly, it is industrially significant to employ an antimony catalyst to produce PET capable of presenting a container rich in heat resistance, and various proposals have heretofore been made. For example, JP-A-7-145233 discloses that PET produced by using trimethylphosphoric acid, magnesium acetate and antimony trioxide, is excellent in transparency and heat resistance and has a small content of acetaldehyde. However, according to the study by the present inventors, this PET does not satisfy a high level of heat resistance required for a container to be used in an application where heat sterilization and filling at a high temperature are required, although the transparency is improved.